The research proposed here deals with the regulation of cholesterol biosynthesis and 3-Hydroxy-3-methylglutaryl coenzyme A reductase in mammalian cells. The highly purified enzyme from rat liver will be purified further to homogeneity and also be used to raise monoclonal rat-anti-mouse antiserum in mouse spleen cells. To study regulation of the enzyme Chinese Hamster Ovary (CHO) cells will be utilized and characterized for their sterol synthesizing capacity. Simultaneously, attempts will be made to select to Compactin resistant CHO mutant. Since cholesterol is a ubiquitous component of all cell membranes, studies on it influence in regulating activities of membrane bound enzymes such as Acyl-CoA cholesterol acyl transferase and HMG-CoA reductase, through changes in the physical properties of membranes will also be continued.